1. Field of Invention
The field of the present invention relates to the multimodal analysis of a small spot on a sample by spectroscopy and sample image analysis. Specifically, it relates to the spectroscopic analysis of the sample using a fiber optic interfaced spectrometer.
2. Prior Art
There has always been an impetus to build spectrometric equipment that would bring radiation to the sample outside the sample compartment of a spectrometer. Dedicated analyzers were and still are being developed that integrate the functions of a spectrometer and a probe in a single package, for example U.S. Pat. No. 6,181,427 B1 (2001) issued to Yarussi, and U.S. Pat. No. 5,659,397A (1997) issued to Miller.
Recent advances in opto-electronics enabled the construction of small hand held full-featured UV-VIS-NIR spectrometers that spectrally analyze radiation brought in by standard fiber optics cables. Two types of fiber optic diffuse reflectance probes exist for use with fiber optic spectrometers: wand like fiber optic probes and integrating spheres.
The most common method for making the wand like fiber optics probes is to use one or more fibers in a bundle to bring light to the sample. The reflected light is collected by other fibers in the bundle and returned to the spectrometer for analysis.
A huge variety of different probes of this general type exists. The basic parameters, such as the number of fibers used to bring light to the sample, the number and position of fibers used to collect the reflected light, fiber packing, diameters, positions, end shapes, are all varied in order to optimize a particular aspect of the analysis. An example is described in U.S. Pat. No. 6,571,118 B1 (2003) issued to Utzinger. These wand-like probes are relatively simple and generally don't require any mirrors. Another variation of the wand like probes that uses light guides and lenses for the interface with a sample is described in U.S. Pat. No. 5,818,996 (1998) issued to Doyle.
The shortcomings of wand like probes are that they obscure the exact sampling spot. This makes sampling spot selection difficult and hence the precise position of the sampled area is somewhat uncertain. Another shortcoming of these probes is that they are subject to a numerical aperture restriction due to optical fibers that limits the amount of light that can be carried to and from the sample. Also, their spectral range is limited by the fibers used.
Wand-like fiber optics probes work well for certain applications and for spectral regions for which suitable optical fibers exist, such as the visible and near infrared spectral regions. In the infrared region no such fibers exist and, in some extreme cases, such as U.S. Pat. No. 5,088,821 (1992) issued to Milosevic, elaborate light pipe systems were built to bring light to sampling stations several meters away from the spectrometer. Several of the sampling stations had integral detectors and returned the electrical signal back to the spectrometer.
Fiber optics limits the numerical aperture of the light that it carries. Only the portion of the diffusely reflected light that can be carried by a fiber back to the spectrometer can be utilized for detection. Following the concept disclosed in U.S. Pat. No. 5,088,821 (1992) issued to Milosevic, one diffuse reflectance probe (M. Milosevic and V. Milosevic, “The Video Barrelino”, Proc. IRUG6, 284-287, 2004) sought to overcome this limitation by incorporating the detector directly into the probe. An integral detector eliminates the need to return the diffusely reflected light back to the spectrometer. By eliminating the need to carry the light to the detector via an optical fiber a much larger amount of diffusely reflected light could be brought to the detector. However, this probe is not usable with fiber optics spectrometers since it returns an electrical signal, not radiation, back to the spectrometer.
Diffuse reflection probes, based on the well known integrating sphere concept, were developed for use with fiber optics spectrometers and are commercially available. Some of these probes employ an integrated radiation source to illuminate the sample. The reflected radiation is returned via fiber optics back to the spectrometer. These probes are made for a very specific purpose. They are constructed to enable the measurement of the total reflectance of a diffusely reflecting sample. They analyze large sample spots—typically around 25 mm in diameter. They provide spatially and directionally uniform irradiation of the sampling area, and are designed to integrate the reflectance from large uneven samples where local unevenness is not of interest and the average value of the reflectance is all that matters. Samples such as grain are analyzed for water, fat, fiber, protein, etc. content since grain to grain variation is not important. Smaller spots can generally be analyzed by aperturing the sampled area. This sample size reduction however is achieved at the expense of the amount of radiation collected from the sample. These probes are thus not adequate for use with small (less than 1 mm in diameter) sample spots.